Many readers challenge themselves with books in other languages. Usually they have to install a translation dictionary in order to catch the nuances, but there are other options.
While instant translation is nothing new, most current examples involve mechanical translation of the text; an article is fed through either Google or Bing Translate and the result is spat out - errors and all.
Duolir goes one step better. This platform offers dual-language ebooks which have been specially made so you can click on each sentence and read the sentence in another language.
In practice it looks something like this:
That's a nifty trick, but it's not exactly a new one.
Google does something similar with Google Translate, and you can also find ebooks like the ones in Duolir on another site, DoppleText. Those ebooks work in (almost?) any app which supports Epub. They also reportedly work on the Kindle, but I would test that first.
If you would like to see a similar trick in action, ReadBeyond.it posted a demo ebook. It doesn't work quite the same as Duolir, but it does work in iBooks and in the Menestrello app. (Thanks, Alberto!)
The ReadBeyond.it translations look like this:
This is a pretty well-known trick, so you're probably wondering why it's not more widely used.
While Duolir offers around 50 titles, and Doppletext has around 40 public domain titles, that's just a drop in the bucket in comparison to the hundreds if not thousands of books where this could be used.
If I had to guess, I'd bet that the cost of making the ebook is probably the killer. There isn't a huge market for this feature, not compared to releasing two separate single language ebooks, so it is difficult to justify the expense.
And that's a shame, because on a technical level this is nifty.